1. Field of the Invention
The present invention relates to a color image processing apparatus and method adaptable to a printing machine that uses black ink and multiple color inks, and more particularly, to a color image processing apparatus and method capable of forming good color images on ordinary or plain paper with reduced ink bleeding.
2. Description of the Related Art
The color inkjet method ejects a drop of ink liquid from a nozzle for recording images. An apparatus that uses the color inkjet method has a simple structure and makes small noise. Generally, the color inkjet method is suitable for compact printing machines. Generally, the color inkjet method needs coated paper having an ink absorption layer in order to form high quality images with small ink bleeding. When images are reproduced on ordinary paper such as copy paper, ink bleeding is likely to occur at boundaries between different colors.
There is known a method of suppressing intercolor bleeding by enhancing the penetrance of ink to paper. However, enhancement of the ink penetrance causes a problem (fethering) of losing the sharpness of edges of printed lines and characters. This problem is conspicuous particularly for black ink. Taking into consideration the above, it is attempted to use a black ink having comparatively low penetrance and color inks having comparatively high penetrance in order to improve the quality of printed images of lines and characters and suppress intercolor bleeding. However, this attempt fails to avoid bleeding between black ink and any of color inks at boundaries therebetween.
A proposal that intends to eliminate the above problem is described in, for example, Japanese Unexamined Patent Publication Nos. 6-113155 and 10-86503. The proposal employs means for replacing some pixels by other pixels at a boundary between an area formed by black ink and an area formed by color inks. This pixel replacement may suppress bleeding between black ink and color inks so that the image quality can be improved.
However, if the pixel replacement is too much applied at the boundary, another problem may occur. If too many color pixels at the boundary are replaced by blank data, the boundary may be lost. If too many black pixels at the boundary are replaced by color inks, the replaced area will have a brightness lower than that of black ink, so that the image has a higher brightness and does not have an enough density as a whole. This degrades the image quality. As described above, there is a possibility that the image quality may be degraded if the pixel replacement is too much applied at the boundary.
Another proposal to avoid bleeding between black ink and color inks is described in Japanese Unexamined Patent Publication No. 10-67125. According to this proposal, black ink and color inks are subject to separate scans for printing. There is a time interval between print with black ink and print with color inks. The time interval effectively suppresses bleeding. However, the printing speed is greatly reduced.
Japanese Unexamined Patent Publication No. 11-77992 proposes to count a recording density from image data and select a suitable one of recording modes on the basis of the recording density. The recording density is defined as a ratio of the number of data bits to be recorded by once main scan to the maximum number of dots recordable by once main scan. The recording density is compared with given threshold values. The recording mode is switched based on the comparison result. The multiple recording modes may have mutually different recording speeds. When the relatively low recording mode is selected, bleeding may be improved. However, the relatively low recording mode may be selected for image data in which the possibility of occurrence of bleeding is low although the recording density is high (such an image data is, for example, a picture-based image such as a photograph). Thus, the recording speed will be unnecessarily set low, so that the printing time increases.